This invention relates to sheet feeders, by which is meant devices associated with a xerographic or other reproduction machines (to be referred to as a `copier` in this specification) for feeding sheets of paper or other copy material on demand from a stack of sheets. In such machines it is usual to use a so-called `nudger` roll in frictional engagement with the top sheet of the stack for extracting the top sheet from the stack and feeding it into the nip of a pair of sheet-feed rolls. Sometimes, because of high sheet-to-sheet friction, the next sheets(s) in the stack is dragged by the top sheet into the nip, resulting in two or more partially-overlapping (shingled) sheets being fed to a downstream sheet-using device, such as the engine of a copier. This almost inevitably results in a `paper jam`, disturbing the smooth operation of the copier and requiring the manual intervention of an operator, which reduces the productivity of the copier.
In the past, this problem of `multi-feeds` has been countered by the provision of a retard pad or roll cooperating with the nudger or other sheet-feed roll. The retard pad is designed to bear on the roll, and have a surface of friction material. When a single sheet enters the nip formed by the roll annd pad, the friction between the roll and the sheet is greater than that between the sheet and the pad, so that the sheet is driven by rotation of the roll, sliding over the retard pad as it does so. If two or more shingled sheets enter the nip, then the friction between the sheets is lower than that between the roll and the top sheet, and that between the second sheet and the pad. The second sheet is therefore brought to a halt by frictional contact with the pad, with the top sheet sliding over it until the trail edge of the top sheet has left the nip, whereupon the stalled or retarded second sheet becomes the top sheet and can be fed if the feed roll is still rotating.
It is known from U.S. Pat. No. 4,496,145 to use a coaxial sheet-separating roll coated with frictional material, and a feed roll follower. The follower rotates with the feed roll, while the separator roll is rotated against the sheet-feed direction to prevent paper jams because of multi-sheet feeding.
U.S. Pat. No. 4,627,607 discloses the use of a separator roll adapted to rotate in the opposite direction to a feed roll, and with a chosen torque, to prevent multi-sheet feeds. U.S. Pat. No. 4,801,134 discloses preventing multi-feeds by using a spring-biased paging roll to engage a sheet-feed roll frictionally to apply a sheet-retarding force opposing forward sheet propagation. U.S. Pat. No. 4,822,023 discloses the use of two contra-rotating rolls, of which the upper one acts as the main feed roll, while the lower one applies frictional forces to the underside of a sheet being fed, in order to prevent multi-feeds.
JP-A-61-243 741 discloses the use of a spring-biased stack tray; a movable pick-up roll; a feed roll, and a reverse roll forming a nip with the feed roll to prevent multi-feeds.
U.S. Pat. No. 4,830,353 discloses the use of a biassed stack tray to urge a stack of sheets towards a conveyor roll at a fixed location. Rotation of the conveyor roll feeds a sheet to the nip between a feed roll and a spring-biased pivoted retard block having a layer of frictional material opposing the feed roll. The friction surface forms an acute angle with the radius of the feed roll which passes through the pivot axis of the block, and is effective to prevent multi-feeds. In this specification, the biassing of the stack tray is done by a pair of tension springs, which act along the line of the centre of gravity of the stack, so that the stack can tilt by virtue of its contact with the friction roll.
Accordingly, as an improvement over the above-mentioned systems, the present invention aims at providing a sheet feeder in which the force urging a stack of sheets upwardly into contact with a sheet extractor roll is automatically augmented when there is no sheet in the nip between a sheet-feed roll and an angularly-movable retard member. By judicious choice of spring values, the nip and stack forces can be reduced, resulting in less drag-out, lower force requirements and a reduced occurrence of multi-feeds.